From single scheme to married scheme

I have gotten a resale unit under Single scheme but now i have met my partner in life. We intend to apply a BTO. Im not sure if i should keep the current resale unit and have my partner be the co owner when we are married. Is the loan shared equally from thereon? Or do we have to get a new valuation done? Is there anyone who can share the process?

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If we get the BTO flat, I would have to pay $40k reslae levy and what other considerations should I think of when we get the BTO? Could we dictate the amount of CPF to be used upon purchase leaving more than $20k in CPF? My partner will be 55 yo soon, will his CPF OA be wiped off (only leaving $20k) when we get the BTO? ANy advise?